I am 23 years old. my heart races, i feel dizzy and short ofbreath while standing, but it gets normal once i sit or lie down. could this be heart failure or coronary blockage?

Orthostatic BP: While not impossible for some type of heart condition, it sounds much more like you are having orthostatic hypotension, which is a drop in your blood pressure when you stand up. It is very common in adolescents and young adults. The symptoms you experience are the results of the low BP - you feel dizzy, and your heart rate increases to try and compensate for the lowering of the bp. See doc to test.

Perhaps POTS: Neither coronary disease nor heart failure are at all likely. It sounds as if your blood pressure drops when you get up. This can be due to dehydration, but if it is a long-standing problem, ask your doctor to consider pots: postural orthostatic hypotension syndrome. Step one is to measure pulse and BP sitting and standing.

Not likely: Sounds like you have possible pots or some form of vas-vagal type scenario . ASHD not likely in age group without heavy smoking history , diabetic history , familial disease states like severely high cholesterol levels. Arrhythmia's should also be looked into but would usually occur at rest and with activity . None the less a stress echo , cardiac event monitor and a tilt table is my rec.